The enhanced transmission system and method proposed in the present invention is especially useful in mobile networks systems using multiple active antennas.
The use of active antennas has improved the performance of the existing telecommunication networks and simplified the deployment of mobile network systems. Moreover the working characteristics of the active antennas allow an improvement of the existing communication methods.
An active antenna is an antenna that contains active electronic components that provide equivalent functionalities to conventional Remote Radio Units (RRU) i.e. containing all necessary Radio Frequency Equipment to ensure the transmission and the reception of 2G, 3G or LTE signals. Typically the active antenna connects to a baseband unit (BBU) by means of a fibre connection. The BBU function can also be integrated in the active antenna with a direct lub connection as input to the active antenna. This allows the construction of antennas of limited size and/or wide frequency range.
Active antennas are usually made of several RF sub-modules, each one integrating RF transmission and reception functions, the active antenna transmitting/receiving a RF signal by combining the signals of each sub-module, usually synchronously, over the air (this synchronization is obtained and controlled by an active antenna controller element 14). That is, the sub-modules operate together (preferably synchronously) for transmission of radiofrequency signals to users of the mobile communication network. Each sub-module comprises a digitally controlled phase shifter (11) (called “static” phase shifter), so the digital signal which feeds each sub-module is phase shifted independently with respect to other sub-modules, a transceiver (TRX) module (12) and an antenna element (13). The TRX module performs transmission and reception functions including functions such as: low power amplification by low noise amplifiers (LNA) in uplink; multicarrier low to medium power amplification (MCPA) in downlink; up/down conversion; digital to analogue conversion; and filtering functions (for example, duplexer functions, separating the uplink and the downlink frequencies in case of FDD). Each TRX is connected to an antenna element (for example a dipole) to transmit over the air an output signal (usually a low-power output signal) as well as receive signals with similar sensitivity, as in a classical macro/RRH deployment.
FIG. 1 shows an active antenna architecture with N sub-modules (or branches) with the digitally controlled phase shifter (11) controlled by the active antenna controller element (14). These phase shifts (represented as ej.Φ1 . . . ej.ΦN, that is, they may be different for each sub-module) are selected as a part of the antenna calibration, that is, the phase shifts (offsets) are selected at the beginning of the antenna operation according to the real working conditions of the antenna (tilt, orientation, . . . ) to achieve proper operation of the antenna. Consequently, the selection of the vector of phase for the N sub-modules is static, that is, the vector of phase is selected (set) at the beginning of the antenna operation and it is fixed (not changed) during the antenna operation, unless a change is needed due to a significant change in the working conditions of the antenna. That is, this phase setting is mainly used for optimization purposes and it is very seldom changed. The antenna tilt settings set through these phase shifters are checked only infrequently (with a check every couple of months, typically) and if it is needed, there is a change in the selected phase shifts. For this reason, the present document characterizes conventional phase shifters as “static” phase shifters: this terminology contrasts with the “adaptive” or “dynamic” phase shifters described later in this document, which typically are changed several times per minute. The intelligence in the active antenna controller to decide which phase vector to apply can either be integrated physically in the active antenna or provided separately in a baseband unit connected (for example, via optical fiber) to the active antenna. However the digital phase shifters can only be located in the active antenna.
The active antenna controller elements control not only the phase shifters but also all the other elements of the different sub-modules (attenuators, filters . . . ) necessary to form the required radiating signal or to receive adequately signals transmitted on the uplink by users.
Another technique frequently used to improve the performance of, for example, 3G wireless networks is the High-Speed Downlink Packet Access HSDPA technology. HSDPA is a packet-based data service in third generation (3G) W-CDMA (Wideband Code Division Multiple Access) systems, which provides high-speed data transmission (with different download rates e.g. 7.2/10.8/16.2/21.6 Mbps over a 5 MHz bandwidth) to support multimedia services.
HSDPA comprises various versions with different data speeds and features. In table 5.1a of the release 9 version of 3GPP TS 25.306, it is shown maximum speeds of different device classes and the combination of features they support.
In order to reach yet higher peak rates (i.e. 28.8 Mbps with 3GPP Release 7), the MIMO (Multiple Input Multiple Output) feature is used in HSDPA.
In MIMO systems, both the transmitter and the receiver are equipped with multiple antennas in order to improve the system performance. In particular, the use of MIMO systems represents a useful solution for improving the capacity and user throughput performance of the networks. The basic MIMO feature as standardised in 3GPP Release 7 is based on two transmitter antennas (at the node B or base station) and two receiving antennas (at the UE) using a single carrier. At the transmitter, the data can be split into one or two data streams and transmitted through the two antennas using the same radio resource (i.e. same transmission time interval and HSDPA codes). In a generic downlink transmitter structure to support MIMO operation the primary and secondary transport blocks are each processed (channel coding and interleaving), then spread and scrambled, subsequently weighted by precoding weights. The resulting channels after MIMO precoding (i.e. MIMO channel#1 and MIMO channel#2) are mapped onto P-CPICH and S-CPICH (Primary and Secondary Common Pilot Channels), respectively before being mapped to the first and second physical antennas respectively. The two streams of data are recovered by the UE from the signals received via its two Rx antennas. Thus, for the MIMO feature to work both the network and the terminals need to be MIMO-enabled. In order to deploy MIMO and transmit two parallel data streams, two power amplifiers are required per sector (one for each of the two antennas). In order not to use an entire carrier for MIMO only (5 MHz), it is more efficient and practical to use the same carrier for MIMO devices as used for non-MIMO devices (e.g. HSDPA legacy terminals).
MIMO technology is an important step in the evolution of HSDPA, as it provides higher data rates in downlink whilst further improving spectrum efficiency. When introducing MIMO in a system, it is indispensable to have two transmission branches (RF chains), including two power amplifiers each one connected to the physical antenna. In order to optimise the usage of the power resource it is highly desirable to balance the power between the two power amplifiers. Whilst MIMO channels are intrinsically perfectly power balanced, all the remaining channels need to be transmitted with equal power by each power amplifier. To this end, two techniques can be used: a first one is the use of transmission diversity (using “Space Time Transmit Diversity” (STTD) for all non-MIMO channels except for the Synchronisation Channel for which “Time Switch Transmit Diversity” (TSTD) is used). Another technique is referred to as Virtual Antenna Mapping (VAM) in this description, and is discussed hereinafter.
A key requirement is to make sure that the technique used to balance power in the non-MIMO channels allows substantially the same performance as would be achieved with the same energy using a single power amplifier. STTD was defined by 3GPP (Release '99) in order to achieve this. However in practice this feature has been found to affect the performance of certain legacy user equipments. In particular, HSDPA UEs with equaliser receivers can be severely impacted. This is due to the time transformation which is performed by STTD, which is ill-adapted to an optimum equalisation process. Some HSDPA devices have been found to deactivate their equaliser for this reason. Field tests have shown that the impact of the use of STTD on the throughput of data received by an HSDPA category 8 device (especially for a type 2 receiver i.e. a single antenna equalizer receiver) is particularly negative under good and medium radio conditions.
Virtual Antenna Mapping (VAM) is an alternative to the use of the STTD technique: VAM is aimed at solving this issue fulfilling above-mentioned requirements. Hence, this technique enables power balancing of the power amplifiers whilst not impacting on the performance of legacy users. The principle of the VAM technique is depicted in FIG. 2. The VAM operation/function 100 can be performed as a baseband function after the mapping onto physical channels for Rel'99 and HSDPA and after precoding for MIMO. The VAM operation/function can also be implemented in logic in a radio unit such as a Remote Radio Head (RRH). The signals shown at the input of adding operations 150, 180 are, for example, the following: Rel'99 refers to the dedicated channel (DCH) which can carry voice or data traffic; HS refers to HSDPA SIMO (Single Input Multiple Output, i.e. HSDPA without MIMO); MIMO Channel #1 (101) is the resulting channel after MIMO precoding operation consisting of the sum of the primary data stream and the secondary data stream weighted with their correspondent weights; and MIMO Channel #2 (102) is the resulting channel after MIMO precoding operation consisting of the sum of the primary data stream and the secondary data stream with their correspondent weights. Said resulting channels after MIMO precoding (i.e. MIMO channel#1 and MIMO channel#2) are mapped (150, 180) onto Primary (103) and Secondary (104) Common Pilot Channels (P-CPICH and S-CPICH)
VAM consists of mapping input signals onto the physical antennas with specific weights for each path. VAM can be seen as a matrix of four weights ω1,ω2,ω3,ω4 and two adders 110 applied to two input signals fed by “virtual antennas” 160, 170 corresponding to the physical antennas of the MIMO operation. The force of the virtual antenna concept is that the UE behaves as if the signals present at the virtual antennas are the ones actually transmitted, although the physical antennas (120, 130) radiate something different. The legacy UE (not supporting MIMO) will only ‘see’ the virtual antenna 160. Whilst its signal will be transmitted on both physical antennas the UE receiver will act as if transmitted from one (the mapping between virtual and physical antennas is transparent for the user equipment). The configuration received by the legacy user is the same as in a single antenna transmission system, the user equipment is not configured for any form of transmit diversity at RRC level. The MIMO UE will see both virtual antenna 160 and virtual antenna 170 and is unaware of the mapping between the virtual and physical antennas, which is transparent to the MIMO operation.
The four weights from the VAM matrix are differentiated by phases only as equal amplitude is required to achieve power balancing between the two physical antennas 120,130. A first power amplifier 140 and a second power amplifier 190 are configured for amplifying the output signals after the VAM function before they are radiated by the physical antennas 120,130. The weights of the VAM matrix are fixed. They are configured for the whole cell and set by Operation & Maintenance (O&M) and typically not changed very often. The VAM weights fulfil totally different objectives than the MIMO precoding weights—the latter ones being variable weights (that can change every 2 ms) used only for the purpose of the MIMO transmission whilst VAM applies to all channels and has as objective to fulfil the two requirements highlighted above.
A pure SIMO mode can be also seen as a particular case of the VAM application, where the second virtual antenna has zero values in the matrix so that the same signal from the primary virtual antenna is mapped on the two antennas but with a given phase offset (that is ω3=ω4=0)
From the legacy user point of view the VAM technique is like a single antenna transmission, i.e. the user terminal demodulates the HSDPA signal as if there were no Transmission diversity in the system. Seen from the transmit side for legacy non-MIMO users, VAM amounts to transmitting the same signal (common channel, Rel'99 and HSDPA non-MIMO) on the two transmit antenna ports but with a different phase.
However, from extensive field testing of VAM functionality (measurements over a large amount of static points which statistically shows the impact of VAM), the following results have been obtained:                When there is no concurrent HSDPA and active MIMO user equipments e.g. only HSDPA (non-MIMO) user equipments in the cell, VAM has little or no impact on HSDPA performance i.e. throughputs observed of HSDPA user equipments with VAM active are nearly the same as the throughputs of HSDPA without VAM (single antenna transmission as in most 3G networks today).        The performance of MIMO with VAM is also very similar to the performance of MIMO with Tx diversity (STTD).        However whenever there is concurrent HSDPA and MIMO traffic, it has been observed that the performance of HSDPA legacy devices is impacted negatively by around 10% for a legacy type 3 device (Rx diversity and equalizer implemented in receiver) and by around 15-20% for a legacy type 2 HSDPA device (no Rx diversity, only equalizer implemented in receiver) whenever the secondary pilot is present in the second virtual antenna and more degradation is observed whenever the MIMO user is fully active with continuous downloads.        
Hence, it is shown that even though the VAM technique has a better performance than previously used techniques such as STTD, it has still a negative impact in HSDPA legacy devices when there is concurrent HSDPA and MIMO traffic.
Enhanced Virtual Antenna Mapping (as disclosed in copending European patent application having application number EP 10382262.3) is a technique which allows enhancement of the VAM performance, improving the user throughput for all HSDPA users wherever deployed in a cell with dual Passive Antennas. The E-VAM solution has been designed and optimized for Macro and Radio Remote Head type deployments.
This latter technique alone lacks flexibility to permit adjustment of the transmit polarization and is somewhat limited, especially in a multiuser scenario.
There is therefore a need for transmission schemes which further improve the performance (especially for legacy HSDPA devices in concurrent HSDPA-MIMO traffic and in a multiuser scenario).